1. Field of the Invention
The present invention relates to a motor-driven shutter for digital still cameras enabled to photograph by using a small-diameter aperture instead of a proper exposure aperture by the action of two relatively operating shutter blades.
2. Description of the Related Art
Generally, there are two kinds of lens shutters for digital still cameras, which are called “a normally closed shutter” and “a normally open shutter”. Between these shutters, the normally open shutter is adapted so that when a power supply switch is closed, shutter blades are in a fully opened state, and an image of an object can be observed on the screen of a monitor, and that when the release button of the camera is depressed, photographing is started by supplying a photographing start signal to an image pickup device. Further, usually, the photographing is finished when an exposure aperture is closed by causing the shutter blades to perform a closing operation according to a photographing end signal, instead of supplying the photographing end signal to the image pickup device. Thereafter, upon completion of transfer of an imaging signal to a storage device during the closed state of the shutter blades, the shutter blades are reset to a fully opened state again. Thus, the camera gets ready for the next photographing. Additionally, a sequence of such operations of the shutter blades is performed by a motor.
Meanwhile, among digital still cameras, there is known a camera having a diaphragm blade. In this case, there are two types of diaphragm mechanisms. One is adapted to have a plurality of diaphragm blades and enabled to continuously change the diameter of an exposure aperture. The other is adapted to selectively enter a preliminarily prepared aperture of a predetermined diameter into a proper exposure aperture. In either of the cameras respectively having the diaphragm mechanisms of these types, there are two manners of performing an exposure aperture control operation. One is to start an exposure aperture control operation in a stage in which a power supply switch is closed. The other is to perform the exposure aperture control operation after the release button is depressed, and before photographing is started. Moreover, recently, in most of the cameras, a diaphragm mechanism is actuated by a motor.
Furthermore, there has been developed a motor for actuating the shutter blade and the diaphragm blade in this manner, which is previously called “an iris motor” and recently called “a moving magnet motor”. This motor is configured so that an output pin (or driving pin) extending in parallel with a rotation shaft of a rotor constituted by a permanent magnet (usually, a two-pole permanent one) is integrally formed therewith in a radial position of the rotor, and that the rotor can be turned by an angle within a predetermined range in a direction corresponding to a direction in which a current is supplied to a stator coil. Further, this motor can be manufactured at low cost and miniaturized still more and reduce power consumption, as compared with a stepping motor.
Although a moving magnet motor is advantageous in cost reduction, miniaturization, and power consumption reduction over a stepping motor, it is disadvantageous for the cost reduction and miniaturization of the camera that a motor for actuating the shutter blade is provided separately from a motor for actuating the diaphragm blade. Thus, preferably, it is sufficient for a camera to have only a single moving magnet motor. Moreover, desirably, the shutter blade is enabled to serve as the diaphragm blade, instead providing the shutter blade and the diaphragm blade separately from each other. Thus, there are expectations for the emergence of a shutter preferably configured so that two shutter blades relatively actuated by a single moving magnet motor serve as diaphragm blades, that the stopped state the shutter blades is maintained at an exposure aperture regulating position corresponding to a small-diameter exposure aperture instead of the proper exposure aperture at least during photographing since a moment just before the photographing is started, and that the shutter blades are closed from the exposure aperture regulating position when the photographing is finished. The present invention relates to a shutter for digital still cameras having such configuration.
Meanwhile, to actuate the shutter blades serving as the diaphragm blade by a single moving magnet motor, the shutter should be adapted so that the shutter blades are reliably stopped at least at three positions, namely, a fully opened position, an exposure aperture regulating position, and a closed position. This is achieved, namely, these positions, at which the shutter blades are stopped, are reliably maintained by satisfying the following requirements. That is, among the three positions, at the fully opened position, the stator coil of the motor is energized so that the shutter blades are operated in an opening direction in which these blades are opened. Further, at the closed position, the stator coil is energized so that the shutter blades are operated in a closing direction in which these blades close. Moreover, at each of the three positions, the rotor or each of the shutter blades is made to abut against a stopper.
However, such a shutter has a drawback in that when the energization the stator coil is once interrupted, the rotor can freely rotate and thus, the position of the rotor is not determined. Therefore, to reliably maintaining the fully opened position and the closed position of the rotor even when the energization of the stator coil is interrupted, it is sufficient that a plurality of magnetic material members are arranged in such a manner as to face the peripheral surface of each of the magnetic poles of the rotor, that an attractive force acting between the magnetic material members due to the magnetic force of the rotor provides torque to the rotor, and that the stopper prevents the rotor from rotating. Further, in the case of the shutter configured in such a manner, it is sufficient to energize the stator coil only when the shutter blades are actuated. Thus, this shutter has an advantage in that the power consumption can be reduced.
Further, even in the case that the stopped state of the shutter blades can be maintained at the fully opened position and the closed position by configuring the shutter in such a manner, it is very difficult to reliably stop each of the shutter blades at the exposure aperture regulating position. That is, as described above, in the case of using this kind of the motor, when the energization of the stator coil thereof is interrupted, the stopped position of the rotor cannot be determined. Thus, it is considered that the stopped state of the rotor is maintained at the exposure aperture regulating position by energizing the stator coil thereby to provide torque to the rotor and by causing the spring member to prevent the rotor from rotating. However, in such a case, the energization of the stator coil should be continued so as to maintain the exposure aperture regulating position. Furthermore, when the shutter blades are closed from the exposure aperture regulating position, the shutter blades should be operated against the pushing force of the spring member. Thus, this requires a larger amount electric power than that required to turn the rotor from the fully opened position to the exposure aperture regulating position. Consequently, this shutter has a drawback in that the power consumption is increased.
Thus, it is considered as another method that the position of the rotor is regulated by sandwiching a driving pin, which is integrally formed with the rotor, between two spring members respectively having pushing forces that act in different directions, or between both end portions of a torsion spring. However, in such a case, the rotor should be operated against the pushing force of each sprig both when the shutter blades are caused to perform the opening operation, and when the shutter blades are caused to perform the closing operation. Therefore, the power consumption of the shutter is not sufficiently reduced on the whole. Furthermore, in such a case, it is difficult to manufacture the shutter so that the rotor reliably stands still at the predetermined position. Even in the case that the shutter can be manufactured so that the rotor can stand still at the predetermined position, the rotor cannot be reliably held at the predetermined position owing to the fitting tolerance of a connecting portion between the driving pin of the rotor and the shutter blade.